The University of Washington Center on Outcomes Research in Rehabilitation (UWCORR) has the broad, long-term objectives of providin.q clinical researchers with better tools or methods to measure patientreported outcomes (PRO) in children and adults with chronic disease and disability by using modern measurement theory and other sophisticated approaches to improve instruments and approaches used to measure pain and fatigue, and understanding of associations between pain and fatigue, and their impact on important outcome variables. The independent research project proposes to conduct two studies. Study 1 is a cross-sectional study of pain and fatigue and their impact on important outcome measures (participation, depression, and disability level) in 1,200 adults and children. Study 2 is a prospective, longitudinal study with 300 adults and children assessed on the set of core measures over six, six month intervals. We will use item response theory modeling framework and other appropriate methods to refine the items and scales, and to propose a set of improved core measures/items for assessment of pain and fatigue in adults and youth with disabilities. We will also test the associations between pain and fatigue, and their relationship to depression, level of disability, and participation in activities using both cross-sectional and longitudinal data. Better understanding of the association of pain and fatigue with important outcomes will allow us to select those items/scales that are most strongly associated with the changes in the outcome variables. Data from Study 2 (longitudinal) will be used to identify reliable subgroups within and across different disability groups characterized by trajectories with a certain pattern of pain and fatigue symptoms. This will inform the final selection of items/scales most relevant to each of the subgroups identified as well as research on treatment and management of pain and fatigue. We propose that PROMISE adopt the following two concepts as Network Projects: (1) develop a psychometrically valid, dynamic system to measure participation in activities for individuals with a broad range of disabilities; utilize the universal design principles to ensure that item banks, CAT system, and supporting materials are fully accessible to researchers, participants, and other constituents, including those with disabilities.